Various non-surgical means have been employed in the therapeutic treatment of living tissue. Such techniques have included the application of ultrasonic energy, electrical stimulation, high frequency stimulation by diathermy, X-rays and microwave irradiation. Techniques such as electrical stimulation, diathermy, X-ray and microwave radiation have shown some therapeutic benefit for soft tissues. However, their use has been somewhat limited because of tissue damage caused by excessive thermal effects. Consequently, the energy levels associated with therapeutic treatments involving diathermy, X-ray, microwave and electrical stimulation have been limited to such low levels that little or no benefit has been obtained. Moreover, the dosage or exposure to microwaves and X-ray radiation must be carefully controlled to avoid radiation related health problems. Ultrasonic energy is non-preferentially absorbed and affects all of the surrounding tissue.
Optical energy generated by lasers has been applied for various medical and surgical purposes because of the monochromatic and coherent nature of laser light which can be selectively absorbed by living tissue depending upon certain characteristics of the wavelength of the light and properties of the irradiated tissue, including reflectivity, absorption coefficient, scattering coefficient, thermal conductivity and thermal diffusion constant. The reflectivity, absorption coefficient and scattering coefficient are dependent upon the wavelength of the optical radiation. The absorption coefficient is known to depend upon such factors as interband transition, free electron absorption, grid absorption (phonon absorption), and impurity absorption, which are dependent upon the wavelength of the optical radiation.
In living tissue, water is a predominant component which has an absorption band according to the vibration of water molecules in the infrared range. In the visible range, there exists absorption due to the presence of hemoglobin. Further, the scattering coefficient in living tissue is a dominant factor.
Thus, for a given tissue type, the laser light may propagate through the tissue, substantially unattenuated, or may be almost entirely absorbed. The extent to which the tissue is heated and ultimately destroyed depends on the extent to which it absorbs the optical energy. It is generally preferred that the laser light be essentially transmissive in tissues which are desired not to be affected, and absorbed by the tissues which are to be affected. For example, when applying laser radiation in a tissue field which is wet with blood or water, it is desired that the optical energy not be absorbed by the water or blood, thereby permitting the laser energy to be directed specifically to the tissue to be treated. Another advantage of laser treatment is that the optical energy can be delivered to the treatment tissues in a precise, well defined location and at predetermined, limited energy levels.